Human mankind is encountered with increasing problems of municipal and industrial waste waters which must be purified and reused. Physical, chemical, and biological purification processes have been developed.
Electroflotation and electroflocculation are established and rather commonly used purification methods especially for industrial waste waters. Direct current is applied to achieve decomposition of water to produce hydrogen, oxygen and other gases depending on the inorganic content of water. Electricity causes various secondary reactions in the solutions which affect the solubility of colloidal particles to decrease. They form flocks which are bound to gas bubbles and lifted by the buoyance forces to the surface to form relatively stable flocks (froth) which is then collected. The present state of the art is described, for example, in the publication by Anglada et al. (2009), J. Chem. Technol. Biotechnol. 84:1747-1755.
Purification and microbial disinfection of waste waters are interconnected tasks. Removal of organic load from waste water usually removes also microbes. The allowed amount of microbes or viruses in purified discharge water is very low and water purification enough carefully to attain the microbial purity is rarely economical. Therefore, microbial residues may need to be disinfected.
When the treated water is released to the natural water circulation, it almost always still contains microbes, particles, and chemicals interfering with the natural habitats. The risk of infections by micro-organisms such as bacteria, viruses, fungi, protozoa, prions, and algae is still high. They may have survived through the purification process or even been propagated in the process itself. Pathogens may cause aberrations in the ecosystems although they will not directly infect humans. The possible harmful effects largely depend on the climatic and soil/drainage basin conditions of the waste discharge environment. However, even occasional infections may be harmful and it needs to secure that the discharge liquids are microbiologically safe by applying disinfection.
The disinfection of contaminated waters can be divided into chemical and physical techniques. The chemical ones include treatment with active chlorine compounds. Ozone is popular in disinfection of drinking water. Various methods to produce ozone and chlorine gas or compounds which release them are in use. Chemical methods have the drawback of yielding residues of unnatural chemicals and their reaction products, in addition to the high costs and difficulties in controlled dosing.
Many disinfection methods deploy ultraviolet (UV) radiation to kill microbes. A drawback is its low effectivity and high selectivity to only certain organisms, and the poor penetration of the UV radiation in water, especially in turbid solutions. A further problem is the fouling of the radiation surfaces that demands their constant cleaning. High voltage electric pulses of kV range kill microbes by causing small pores in cell membranes that allows cell contents to leak out. It is evident that different microbes have largely different sensitivities to the electricity. Applying of the high voltages is limited to spaces closed from the public.
Low-voltage direct current has been deployed for disinfection of waste waters in various formats. The most popular is in situ production of oxidative chlorine compounds from concentrated NaCl solutions exemplified by publication WO 2012 170774 A1 (Lumetta M.). The chlorine gases from anode are introduced to water to be disinfected. The gases contain also other disinfecting compounds like reactive oxygen, ozone, alkali and acids. The electrodes may include semipermeable membranes or porous filters so that gases at anode and cathode do not mix as exemplified by publication by Baichen W., CN1075699A. In situ production of disinfection gases has the advantage of production of poisonous chemicals without need of their storage and transport. The main disadvantage of the in situ processes is the use of semipermeable membranes which tend to clog and must be regularly cleaned and/or changed.
The electrochemical cathode reactions produce basic hydroxyl ions and molecular hydrogen. The hydrogen is generated about 0.4 liter (NTP) per Ah. The hydrogen formation is less useful for the disinfection while it is important for obtaining the flotation effect. US 2004/0031761 (M. Blaschke et al.) describes a device without cathodic hydrogen evolution. In proper conditions the reduction power at cathode can be transformed to hydrogen peroxide formation which may be used as a disinfectant. The method is, however, too complex for wider use.
The electric disinfection systems have been tested in various electrolysis chambers including additional walls or not. A classification of different prior art cells is described in FIG. 3 by Angala, A. et al. (2009) J. Chem. Technol. Biotechnol. 84:1747-1755.
Anodic corrosion is a serious problem in electroflotation, as well as, in the disinfection systems. It can be diminished by specific coatings of the electrodes, like boron-doped diamond on titanium or stainless steel as described by V. Schmalz in Water Research 43:2009, pp. 5260-5266. Such anode coatings have been considered to provide higher organic oxidation rates and greater current efficiencies than other commonly used metal oxides like PbO2 and Ti/SnO2—Sb2 O5. High current density increases the generation of electrochemical oxidants. The specific coatings of the electrodes provide relatively limited benefits considering the increased costs of the electrodes. Titan anode was coated with nanocatalytic TiO2 of 10-35 nm thickness by S. Zhang (WO2012088867 A1). Because of the stability of the grainy nano-coating is dependent on the polarity of the electrode, the polarity cannot obviously be changed to prevent the clogging of electrodes which makes a drawback in the said invention. Clogging is a general serious drawback in the use of such coatings.
Electrode corrosion is also exploited in waste water purification. When corroding (synonyms: reactive, dissolving, consumable, sacrificed etc.) electrodes are exploited in a flotation device construction, the purpose is to produce coagulating metal ions (e.g. of Fe, Al, Mg). This has advantage of unnecessity to add them whereas a very serious drawback is that the control of the process is lost since the amount of metal dissolution and the relative gas evolution cannot be optimized. Limitations to change polarity are also met. Reactive electrodes have been, however, described with different constructions in the context of flotation devices exemplified by U.S. Pat. No. 7,914,662 B” (2011) by Robinson, V. N. E. The publication describes Al or Fe reactive anodes with inert cathode. US 2012/0186992 (Barrack, A.) describes corroding electrodes which additionally contain strong agitation of the fluid to force flocculating materials to meet. U.S. Pat. No. 3,975,247 (Stralser, B. J., 1976) exploits multiple of electrodes which are not electrically connected to a DC power supply except the outer and inner electrodes. The essential physical effect of the inner metal plates is to act as barriers to force waste water to flow up to down and cause mixing which may aid in electroflocculation.
Especially serious hindrance for adoption of electric technologies in waste water purification is the technical realization of convenient and long-life maintenance-free equipment. The primary problems arise from electrode corrosion and electrode passivation by deposition of materials on the electrodes. When considering the overall economy, usual low-corrosion anode materials, like titanium and stainless steel may be optimal if the corroded electrodes can be easily serviced, i.e. changed and/or cleaned.
The present invention exploits the low voltage electricity techniques but avoids the above-described drawbacks by using a novel integrated construction for flotation and disinfection device. The buoyance force of electro-generated gas bubbles are arranged to take an optimal role in liquid flow to up and down while the construction remains simple and allows an easy service and easy removal of the corroding parts.